This invention relates to museum type display cases, and in particular, to a new and improved display case with a controlled climate therein, sometimes referred to as a microclimate.
A controlled stable microclimate is desirable for sensitive objects that should be displayed at relative humidity levels and/or temperature levels different than those of the ambient environment. A typical microclimate incorporates three basic elements and the success in providing an ideal environment for a displayed item depends upon the three: a humidity buffering material such as silica gel, a seal, and a hygrometer.
A typical display case for a microclimate has a base and a removable cover with the object on display supported on the base and with the cover transparent for viewing the object. The cover is removable for access to the object and a seal is provided between the cover and base for limiting air flow into and out of the interior of the case. The display cases almost always are rectangular in plan, with square corners.
In one conventional type of display case, an airtight seal between cover and base is sought by caulking a mastic sealant into the join between the cover and the base. At the present time, one of the silicone mastics is preferred. A humidity buffering material, or humidity buffer, typically silica gel, is carried in an open tray in the base for controlling humidity within the case.
Silica gel is an inert amorphous material and in granular form has vast surface area for adsorption and desorption of moisture. After conditioning in an environmental chamber or over saturated salt solutions of a desired relative humidity value, the silica gel can be placed in a sealed case or container and will adsorb and desorb moisture in the container, maintaining the relative humidity within the container at the conditioned value of the silica gel. With a sufficient amount of silica gel and appropriate application, along with an effective seal, longterm buffering stability can be obtained within the container. However, the buffering stability and capacity of the silica gel is compromised by leakage of the container's seals. Outside air leaking into the container will cause the silica gel to equilibrate with the changed environment resulting in the requirement for reconditioning the silica gel to the desired relative humidity level in the manner described above.
Each time access to the case is required for servicing the humidity buffer or calibrating the hygrometer or for access to the object on display, it is required to slice through the sealant and sometimes the join to remove the cover. Then before resealing, the old mastic must be removed from the base and the cover, typically by scraping, in order to obtain a new seal when the parts are again calked and reassembled. This type of seal is not attractive, its use is messy, and it is a time consuming operation to remove an old sealant and apply a new sealant. Sometimes the case is damaged during this operation, and the actual effectiveness of this type of seal is in doubt.
With this type of case, considerable time and effort are required for maintaining the case. Typically the humidity buffer must be serviced every two months and two to three people are required to perform the removal and replacement in a safe manner.
In another conventional type of display case for achieving a controlled stable microclimate, an airtight seal is sought by welding an acrylic frame to the edge of the display cover. A pair of concentric grooves are machined in the frame, with the grooves curving around the corners of the frame and with one continuous length of a seal ring inserted into each groove. The seal is obtained by compressing the cover and the attached frame and rings against the acrylic back or base of the display case. Compression is maintained by placement of screws through the back or base and into the frame between the seal rings. The screws are usually placed at two to three inch intervals.
This type of case is costly due to the labor intensiveness in fabrication of the sealing mechanisms, which must be repeated for each and every case. Servicing this type of microclimate requires removing the case to the conservation laboratory in order to access the hygrometer for calibration and/or the humidity buffer for regeneration and/or the object on display for inspection. Considerable care and time are required to replace the screws in order to prevent the planar surfaces of the acrylic frame and acrylic base or back from warping and in order to ensure that uniform tension is exerted upon the seal rings.
Even when handled with optimum care, this method of seal application is highly imperfect. Firstly, the seal rings are stressed at the location of the adjacent screws even when tension may be uniform on one screw with respect to another screw. These stress points fatigue the seal rings and over time leakage results. Secondly, regardless of the uniformity of tension exerted upon the acrylic frame and base or back, a planar surface will always tend to twist or warp when subjected to compressive force. This is the nature of acrylic sheet regardless of its thickness, and may be likened to grain direction. When pressure is exerted on the cross-grain the warping problems is eliminated.
The seals in such display cases have to be carefully specified and correctly implemented in order to work satisfactorily; the methods and the materials used in forming the seals must be applied correctly. Also, the deep open tray storage of the humidity buffer material is not efficient as the lower material in the tray is not easily accessible to the atmosphere. Maximum area of the humidity buffer should be exposed to the atmosphere for optimum performance. Also it is desirable to have the humidity buffer material out of view.
It is an object of the present invention to provide a new and improved display case which has low maintenance requirements with respect to the case itself, to servicing the humidity buffer, and to hygrometer calibration. A further object is to provide such a display case which is of a design that is readily adapted to cases of various sizes and various configuration, including the most widely used rectangular plan with square corners. An additional object is to provide such a display case which is easy and simple to operate with simple closure mechanisms that do not require high-level technique or extensive time to operate.
It is a particular object of the invention to provide a display case having an upper chamber for display and a concealed lower chamber for the humidity buffer, with separate access to the lower chamber for changing humidity buffer and to the upper chamber for access to the object on display. An additional object is to provide a display case which can utilize conservation-safe materials throughout and which to a substantial degree, utilizes stock items rather than custom fabricated components. A most important object is to provide a display case which meets these various requirements and also is aesthetically acceptable in the envirnonment in which it will be used.
It is an object of the invention to provide a display case incorporating a solid seal construction which is easily closed and opened repeatedly, rather than utilizing the conventional sticky mastic sealant. An additional object is to provide such a case incorporating provisions for adjusting the sealing pressure around the perimeter of the join between cover and base in order to achieve a uniform sealing pressure. These and other objects, advantages, features and results will more fully appear in the course of the following description.